Download or read book Understanding and Overcoming Head Motion in Ultra high Field Magnetic Resonance Imaging with Parallel Radio frequency Transmission written by Alix Plumley and published by . This book was released on 2022 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Non selective Refocusing Pulse Design in Parallel Transmission for Magnetic Resonance Imaging of the Human Brain at Ultra High Field written by Aurélien Massire and published by . This book was released on 2014 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: In Magnetic Resonance Imaging (MRI), the increase of the static magnetic field strength is used to provide in theory a higher signal-to-noise ratio, thereby improving the overall image quality. The purpose of ultra-high-field MRI is to achieve a spatial image resolution sufficiently high to be able to distinguish structures so fine that they are currently impossible to view in a non-invasive manner. However, at such static magnetic fields strengths, the wavelength of the electromagnetic waves sent to flip the water proton spins is of the same order of magnitude than the scanned object. Interference wave phenomena are then observed, which are caused by the radiofrequency (RF) field inhomogeneity within the object. These generate signal and/or contrast artifacts in MR images, making their exploitation difficult, if not impossible, in certain areas of the body. It is therefore crucial to provide solutions to mitigate the non-uniformity of the spins excitation. Failing this, these imaging systems with very high fields will not reach their full potential.For relevant high field clinical diagnosis, it is therefore necessary to create RF pulses homogenizing the excitation of all spins (here of the human brain), and optimized for each individual to be imaged. For this, an 8-channel parallel transmission system (pTX) was installed in our 7 Tesla scanner. While most clinical MRI systems only use a single transmission channel, the pTX extension allows to simultaneously playing various forms of RF pulses on all channels. The resulting sum of the interference must be optimized in order to reduce the non-uniformity typically seen.The objective of this thesis is to synthesize this type of tailored RF pulses, using parallel transmission. These pulses will have as an additional constraint the compliance with the international exposure limits for radiofrequency exposure, which induces a temperature rise in the tissue. In this sense, many electromagnetic and temperature simulations were carried out as an introduction of this thesis, in order to assess the relationship between the recommended RF exposure limits and the temperature rise actually predicted in tissues.This thesis focuses specifically on the design of all RF refocusing pulses used in non-selective MRI sequences based on the spin-echo. Initially, only one RF pulse was generated for a simple application: the reversal of spin dephasing in the transverse plane, as part of a classic spin echo sequence. In a second time, sequences with very long refocusing echo train applied to in vivo imaging are considered. In all cases, the mathematical operator acting on the magnetization, and not its final state as is done conventionally, is optimized. The gain in high field imaging is clearly visible, as the necessary mathematical operations (that is to say, the rotation of the spins) are performed with a much greater fidelity than with the methods of the state of the art. For this, the generation of RF pulses is combining a k-space-based spin excitation method, the kT-points, and an optimization algorithm, called Gradient Ascent Pulse Engineering (GRAPE), using optimal control.This design is relatively fast thanks to analytical calculations rather than finite difference methods. The inclusion of a large number of parameters requires the use of GPUs (Graphics Processing Units) to achieve computation times compatible with clinical examinations. This method of designing RF pulses has been experimentally validated successfully on the NeuroSpin 7 Tesla scanner, with a cohort of healthy volunteers. An imaging protocol was developed to assess the image quality improvement using these RF pulses compared to typically used non-optimized RF pulses. All methodological developments made during this thesis have contributed to improve the performance of ultra-high-field MRI in NeuroSpin, while increasing the number of MRI sequences compatible with parallel transmission.
Download or read book Parallel Transmission for Magnetic Resonance Imaging of the Human Brain at Ultra High Field written by Martijn Anton Hendrik Cloos and published by . This book was released on 2012 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: The focus of this thesis lies on the development, and implementation, of parallel transmission (pTx) techniques in magnetic resonance imaging for flip-angle homogenization throughout the human brain at ultra-high field. In order to allow in-vivo demonstrations, a conservative yet viable safety concept is introduced to control the absorbed radiofrequency (RF) power . Subsequently, novel methods for local SAR control and non-selective RF pulse-design are investigated. The impact of these short and energy-efficient waveforms, referred to as kT-points, is first demonstrated in the context of the small-tip-angle domain. Targeting a larger scope of applications, the kT-points design is then generalized to encompass large flip angle excitations and inversions. This concept is applied to one of the most commonly used T1-weighted sequences in neuroimaging. Results thus obtained at 7 Tesla are compared to images acquired with a clinical setup at 3 Tesla, validating the principles of the kT-points method and demonstrating that pTx-enabled ultra-high field systems can also be competitive in the context of T1-weighted imaging. Finally, simplifications in the global design of the pTx-implementation are studied in order to obtain a more cost-effective solution.
Download or read book Ultra High Field Neuro MRI written by Karin Markenroth Bloch and published by Elsevier. This book was released on 2023-08-31 with total page 626 pages. Available in PDF, EPUB and Kindle. Book excerpt: Ultra-High Field Neuro MRI gives a comprehensive reference and educational resource on the current state of neuroimaging at ultra-high field (UHF), with an emphasis on 7T. Sections cover the MR physics aspects of UHF, including the technical challenges, as well as technical and practical solutions that have led to the rapidly growing interest in 7T MRI. Individual chapters are dedicated to the different techniques that most strongly benefit from UHF, as well as chapters with a focus on different application areas in anatomical imaging, functional imaging, diffusion imaging and metabolic imaging. Finally, given the expected rapid growth in clinical use as sales of now FDA approved 7T are skyrocketing, several chapters highlight the clinical neuro applications for which 7T has proven superior. The book's primary audience includes MR scientists who develop MR technologies and/or support clinical and neuroscience research, as well as users who want to utilize UHF neuro MR techniques in their research, including clinicians and neuroscientists.
Download or read book The Physics of Clinical MR Taught Through Images written by Val M. Runge and published by Thieme. This book was released on 2005 with total page 244 pages. Available in PDF, EPUB and Kindle. Book excerpt: The objective of this textbook is to teach, through images, a practical approach to magnetic resonance (MR) physics and image quality. Unlike prior texts covering this topic, the focus is on clinical images rather than equations. A practical approach to MR physics is developed through images, emphasizing knowledge of fundamentals which is important in achieving high image quality. The text is organized into concise chapters, each discussing an important point relevant to clinical MR and illustrated with images from routine patient exams. The topics covered encompass the breadth of this field, from imaging basics and pulse sequences to advanced topics including contrast-enhanced MR angiography, spectroscopy, perfusion and diffusion. Discussion of the latest hardware and software innovations, such as multichannel-phased array coil technology and parallel imaging, is included as these topics are critical to current and future advances.
Download or read book Magnetic Resonance Imaging at Ultra High Field written by Richard Ely Burgess and published by . This book was released on 2004 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Abstract: Even before the development of magnetic resonance imaging, scientists and engineers repeatedly predicted that, despite the theoretical potential of high field, physical and engineering challenges would prevent the practical realization of gains in signal to noise. Many of the arguments used to disparage high field MRI can be divided into issues of uniform excitation, image distortion, and patient safety. In the former category lies challenges such as RF penetration limitations, dielectric resonances, coil self-resonance, coil-sample interactions, and RF power requirements, which may prevent uniform B1 can best be studied with numerical modeling techniques. Within the second category are effects such as chemical shift artifact, susceptibility distortions, and contrast convergence that can be well studied through analytic techniques and methodical manipulation of imaging parameters. In the category of safety belong RF power deposition and magnetohydrodynamic effects. In this thesis, issues of static field safety will be exhaustively explored and investigation of image contrast and quality will be undertaken to assess the potential of the 8 Tesla system for human neuroimaging. This thesis will specifically examine the theoretical risk of cardiac arrhythmia from induced currents and demonstrate the negligible cardiac, cognitive, and physiological bioeffects through animal and human studies. The extent of signal to noise ratio enhancement possible at 8 Tesla will be assessed and harnessed to obtain high resolution whole brain images. In the end, experimental results and analysis show that, despite the presence of artifact, high resolution images of the human brain with unique contrast can be safely obtained at 8 Tesla.
Download or read book Universal Parallel Transmission Pulse Design for the Human Brain and Spinal Cord MRI at 9 4T written by Ole Geldschläger and published by . This book was released on 2021 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Magnetic resonance imaging is a powerful, non-invasive technology to acquire anatomical images from the human body. Operating at a magnetic field strength of 7T or higher (i.e. ultrahigh field (UHF)) provides a higher signal-to-noise ratio, facilitates higher spatial resolutions, and potentially improves diagnostic sensitivity and specificity compared to clinical field strength, such as 1.5T or 3T. Unfortunately, UHF is accompanied with technical hurdles, from which the most problematic is the inhomogeneity in the radiofrequency transmit field. That can lead to spatially varying flip angles and, thus, to signal dropouts, local brightening or spatially altering imaging contrast. The most flexible approach to address this issue is the parallel transmission (pTx) technique, which itself has the disadvantage of a lengthy calibration procedure. To overcome the calibration procedure the 'universal pTx pulse' (UP) concept was introduced. It is a radiofrequency pulse design concept that relies on a pre-collected design database. The resulting pulses then work on a wide cohort of subjects without recalibration. As a first step, in this PhD project the advantages of imaging the human spinal cord at UHF were exploited. It was possible to acquire the first images from the human spinal cord at an ultrahigh in-plane resolution of 0.15x0.15mm2 at 9.4T. The images showed the tiny structures of the spinal cord in great detail. The signal-to-noise ratio and T2 *- times in the human spinal cord at 9.4T were presented. Furthermore, in this thesis the UP concept was further developed, in order to use UHF and the pTx technique more widely. While UPs were originally introduced for whole-brain or slice selective excitation, in this work a feasibility study for UPs for local excitation in the human brain (i.e. exciting only specific regions of the brain, while others should experience no excitation) was performed. UPs that locally excite the visual cortex area were calculated. The underlying transmit k-space trajectory for these radiofrequency pulses were 'spiral' trajectories. These local excitation UPs were successfully tested in vivo on nondatabase subjects at 9.4T. In a next step, the UP performance was further improved by optimizing the underlying transmit k-space trajectory to match the excitation target. The trajectory optimization and the UP design algorithms have been implemented into an open source software package (called OTUP) and demonstrated using simulations and in vivo experiments at 9.4T. The code was tested for three different target excitation pattern with varying complexity.
Download or read book Magnetic Source Imaging of the Human Brain written by Zhong-Lin Lu and published by Taylor & Francis. This book was released on 2003-10-17 with total page 415 pages. Available in PDF, EPUB and Kindle. Book excerpt: This book is designed to acquaint serious students, scientists, and clinicians with magnetic source imaging (MSI)--a brain imaging technique of proven importance that promises even more important advances. The technique permits spatial resolution of neural events on a scale measured in millimeters and temporal resolution measured in milliseconds. Although widely mentioned in literature dealing with cognitive neuroscience and functional brain imaging, there is no single book describing both the foundations and actual methods of magnetoencephalopgraphy and its underlying science, neuromagnetism. This volume fills a long-standing need, as it is accessible to scientists and students having no special background in the field, and makes it possible for them to understand this literature and undertake their own research. A self-contained unit, this book covers MSI from beginning to end, including its relationship to allied technologies, such as electroencephalography and modern functional imaging modalities. In addition, the book: *introduces the field to the non-specialist, providing a framework for the rest of the book; *provides a thorough review of the physiological basis of MSI; *describes the mathematical bases of MSI--the forward and inverse problems; *outlines new signal processing methods that extract information from single-trial MEG; *depicts the early, as well as the most recent versions of MSI technology; *compares MSI with other imaging methodologies; *describes new paradigms and analysis techniques in applying MSI to study human perception and cognition, which are also applicable to EEG; and *reviews some of the most important results in MSI from the most prominent researchers and laboratories around the world.
Download or read book Engineering Parallel Transmit Receive Radio Frequency Coil Arrays for Human Brain MRI at 7 Tesla written by Jérémie Daniel Clément and published by . This book was released on 2019 with total page 162 pages. Available in PDF, EPUB and Kindle. Book excerpt: Mots-clés de l'auteur: ultra-high field ; phased array ; center-shortened dipoles ; parallel imaging ; parallel transmission ; RF phase shimming ; particle-swarm algorithm ; surface coils ; 7T ; whole-brain imaging.
Download or read book Parallel Magnetic Resonance Imaging written by Florian Wiesinger and published by . This book was released on 2005 with total page 145 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Comprehensive Head Motion Correction For Functional Magnetic Resonance Imaging written by Zahra Faraji-Dana and published by . This book was released on 2017 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Head motion artifacts are major confounds that limit use of functional magnetic resonance imaging (fMRI) in neuroscience research and clinical settings. Prospective motion correction is a promising candidate solution for head motion in fMRI that ideally allows the image plane to remain fixed with respect to the moving head (i.e., in the moving reference frame). Prospective motion correction has been shown to correct successfully for rigid body movement artifacts, but residual geometric distortion due to dynamic magnetic field nonuniformities and dynamic changes in receiver coil sensitivity profiles in the moving reference frame still remain a problem. This thesis focuses on three objectives. First, I investigated and corrected for the influence of respiratory effects on the performance of dynamic geometric correction using Phase Labeling for Additional Coordinate Encoding (PLACE). It was demonstrated that PLACE combined with the dynamic off-resonance in k-space (DORK) method, and temporal averaging substantially improved fMRI data quality in comparison to the results obtained by standard processing and static geometric distortion correction. Second, I verified that appreciable signal artifacts occur due to coil sensitivity changes in fMRI maps in presence of overt head motion with prospective motion correction using Prospective Acquisition CorrEction (PACE) technique [1]. Sensitivity map compensations were shown to suppress these artifacts and provide improved fMRI results Third, I studied signal artifacts resulted from the head motion between the coil sensitivity map measurement (i.e., the calibration step) and data acquisition for fMRI with parallel-imaging reconstruction methods using two parallel imaging schemes: sensitivity encoding (SENSE) and generalized autocalibrating partially parallel acquisitions (GRAPPA) with acceleration factors 2 and 4. Coil sensitivity map compensations were shown to improve fMRI results obtained with PACE in the presence of overt head motion compared to those obtained with no overt head motion. Overall, prospective motion correction, integrated dynamic geometric distortion correction, and coil sensitivity map correction present an appealing compound approach for suppressing rigid and non-rigid motion artifacts during fMRI. This thesis has developed robust and comprehensive head motion correction strategies that ultimately will expand the patient populations for which fMRI can be performed robustly.
Download or read book Susceptibility Effects in Ultra high Field Magnetic Resonance Imaging of the Human Brain written by Trong-Kha Truong and published by . This book was released on 2004 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Abstract: In magnetic resonance imaging (MRI), susceptibility differences between deoxygenated blood or iron and surrounding tissue induce mesoscopic static magnetic field (B0) inhomogeneities that provide a valuable contrast mechanism for imaging of the vasculature, functional MRI, and assessment of iron content. On the other hand, susceptibility differences at air/tissue interfaces induce macroscopic B0 inhomogeneities resulting in image artifacts. Ultra-high field (greater than or equal to 7 tesla) MRI benefits from an enhanced susceptibility contrast, but also suffers from more severe susceptibility artifacts. The development of methods to reduce such artifacts while maintaining susceptibility contrast is the objective of this research. Development of susceptibility artifact correction methods requires knowledge of the macroscopic susceptibility effects, which can be quantified by mapping B0, whereas optimization of methods sensitive to susceptibility contrast requires understanding of the mesoscopic susceptibility effects, which can be characterized by relaxation time measurements. We first developed various methods for B0 numerical simulations and experimental mapping. Our simulations showed that air/tissue interfaces at the shoulders induce substantial B0 inhomogeneities in the brain, and that tilting the head backwards can significantly reduce some of these inhomogeneities. We used the B0 simulations and experimental mapping as well as radiofrequency magnetic field (B1) mapping to correlate the B0 and B1 inhomogeneity with the artifacts observed on images of the human brain acquired at 8 T. We then evaluated different susceptibility artifact correction methods at ultra-high field strength using B0 maps, including passive shimming, post-processing, and gradient compensation, and found the latter to be the most effective. Finally, we developed various methods for T2 and T2* relaxation time measurements at ultra-high field strength that are faster and less sensitive to B0 and/or B1 inhomogeneity than existing methods, and demonstrated these advantages in phantom and human studies. New findings obtained in this work will be used to improve ultra-high field MRI of the human brain, particularly for imaging of the venous microvasculature and assessment of iron content.
Download or read book MRI from Picture to Proton written by Donald W. McRobbie and published by Cambridge University Press. This book was released on 2017-04-13 with total page 405 pages. Available in PDF, EPUB and Kindle. Book excerpt: MR is a powerful modality. At its most advanced, it can be used not just to image anatomy and pathology, but to investigate organ function, to probe in vivo chemistry, and even to visualise the brain thinking. However, clinicians, technologists and scientists struggle with the study of the subject. The result is sometimes an obscurity of understanding, or a dilution of scientific truth, resulting in misconceptions. This is why MRI from Picture to Proton has achieved its reputation for practical clarity. MR is introduced as a tool, with coverage starting from the images, equipment and scanning protocols and traced back towards the underlying physics theory. With new content on quantitative MRI, MR safety, multi-band excitation, Dixon imaging, MR elastography and advanced pulse sequences, and with additional supportive materials available on the book's website, this new edition is completely revised and updated to reflect the best use of modern MR technology.
Download or read book Motion robust Pulse Design for Parallel Transmission Excitation at Ultra high Field MRI written by Luke Watkins and published by . This book was released on 2023 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: This note is part of Quality testing.
Download or read book Fundamental and Practical Limits to Image Acceleration in Parallel Magnetic Resonance Imaging written by Michael A. Ohliger and published by . This book was released on 2005 with total page 320 pages. Available in PDF, EPUB and Kindle. Book excerpt: Imaging speed in conventional magnetic resonance imaging (MRI) is limited by the performance of magnetic field gradients and the rate of power deposition in tissue. Parallel MRI techniques overcome these constraints by exploiting information stored within the spatial sensitivity patterns of radiofrequency detector arrays to substitute for some of the spatial information that would normally be obtained using magnetic field gradients. Parallel MRI strategies have been applied clinically to increase patient comfort, enhance spatial resolution, expand anatomical coverage, and reduce image artifacts. The effectiveness of parallel MRI techniques is largely determined by the amount of spatial information that is stored in the detector coil sensitivities. This dissertation investigates the spatial encoding properties of coil arrays from three practical and fundamental perspectives. First, a novel array design is presented that enables spatial encoding in multiple directions simultaneously. Second, the impact of inductive coupling between array elements in parallel MRI is investigated theoretically and experimentally. Finally, electromagnetic calculations are described that permit computation of the ultimate intrinsic signal-to-noise ratio available to any physically realizable coil array for parallel MR. These calculations help to establish fundamental limits to the image accelerations that may be achieved using parallel MRI techniques. These limits are intrinsically related to the wavelengths of the electromagnetic fields at MR imaging frequencies. The sensitivity patterns that correspond to the ultimate intrinsic SNR also represent potential starting points for new coil designs.
Download or read book Magnetic Resonance Imaging written by Robert W. Brown and published by John Wiley & Sons. This book was released on 2014-06-23 with total page 976 pages. Available in PDF, EPUB and Kindle. Book excerpt: New edition explores contemporary MRI principles and practices Thoroughly revised, updated and expanded, the second edition of Magnetic Resonance Imaging: Physical Principles and Sequence Design remains the preeminent text in its field. Using consistent nomenclature and mathematical notations throughout all the chapters, this new edition carefully explains the physical principles of magnetic resonance imaging design and implementation. In addition, detailed figures and MR images enable readers to better grasp core concepts, methods, and applications. Magnetic Resonance Imaging, Second Edition begins with an introduction to fundamental principles, with coverage of magnetization, relaxation, quantum mechanics, signal detection and acquisition, Fourier imaging, image reconstruction, contrast, signal, and noise. The second part of the text explores MRI methods and applications, including fast imaging, water-fat separation, steady state gradient echo imaging, echo planar imaging, diffusion-weighted imaging, and induced magnetism. Lastly, the text discusses important hardware issues and parallel imaging. Readers familiar with the first edition will find much new material, including: New chapter dedicated to parallel imaging New sections examining off-resonance excitation principles, contrast optimization in fast steady-state incoherent imaging, and efficient lower-dimension analogues for discrete Fourier transforms in echo planar imaging applications Enhanced sections pertaining to Fourier transforms, filter effects on image resolution, and Bloch equation solutions when both rf pulse and slice select gradient fields are present Valuable improvements throughout with respect to equations, formulas, and text New and updated problems to test further the readers' grasp of core concepts Three appendices at the end of the text offer review material for basic electromagnetism and statistics as well as a list of acquisition parameters for the images in the book. Acclaimed by both students and instructors, the second edition of Magnetic Resonance Imaging offers the most comprehensive and approachable introduction to the physics and the applications of magnetic resonance imaging.
Download or read book Multimodal Imaging in Neurology written by Hans-Peter Müller and published by Springer Nature. This book was released on 2022-06-01 with total page 75 pages. Available in PDF, EPUB and Kindle. Book excerpt: The field of brain imaging is developing at a rapid pace and has greatly advanced the areas of cognitive and clinical neuroscience. The availability of neuroimaging techniques, especially magnetic resonance imaging (MRI), functional MRI (fMRI), diffusion tensor imaging (DTI) and magnetoencephalography (MEG) and magnetic source imaging (MSI) has brought about breakthroughs in neuroscience. To obtain comprehensive information about the activity of the human brain, different analytical approaches should be complemented. Thus, in "intermodal multimodality" imaging, great efforts have been made to combine the highest spatial resolution (MRI, fMRI) with the best temporal resolution (MEG or EEG). "Intramodal multimodality" imaging combines various functional MRI techniques (e.g., fMRI, DTI, and/or morphometric/volumetric analysis). The multimodal approach is conceptually based on the combination of different noninvasive functional neuroimaging tools, their registration and cointegration. In particular, the combination of imaging applications that map different functional systems is useful, such as fMRI as a technique for the localization of cortical function and DTI as a technique for mapping of white matter fiber bundles or tracts. This booklet gives an insight into the wide field of multimodal imaging with respect to concepts, data acquisition, and postprocessing. Examples for intermodal and intramodal multimodality imaging are also demonstrated. Table of Contents: Introduction / Neurological Measurement Techniques and First Steps of Postprocessing / Coordinate Transformation / Examples for Multimodal Imaging / Clinical Aspects of Multimodal Imaging / References / Biography
